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In solids, molecules are the most closely packed together compared to liquids and gases. This is due to the strong intermolecular forces present in solids, which hold the molecules in fixed positions within a defined structure. The arrangement of particles in a solid is typically in a regular, ordered structure, such as a crystalline lattice, where the particles vibrate around fixed positions but do not move freely. This close packing results in solids having a definite shape and volume.

In contrast, liquids have molecules that are less tightly packed than those in solids, allowing them to flow while still maintaining a relatively fixed volume. In gases, the spacing between molecules is much greater, with particles moving freely and independently, resulting in neither a definite shape nor volume. Therefore, solids are characterized by their close molecular arrangement, making them the phase of matter with the most tightly packed molecules.